The present invention relates to a hydraulic system. More particularly this invention concerns the control of a hydraulic pump.
Hydraulic pumps are known having a pressurizable port and an outlet. The pump generates a pressure at its outlet which is generally proportional to the pressure at its control port. A typical such arrangement is a vane-type pump with a rotor that is radially displaceable by a pressurizable pilot cylinder, or an axial-piston pump whose swash plate can be tipped against the force of a spring or against the force of a hydraulic cylinder by means of a pilot cylinder.
When such devices are connected to a load it is known to control the pressure produced by the pump by means of simple overload devices which depressurize the pressurizable pilot port whenever the pressure in the outlet line exceeds a predetermined maximum. This ensures that if the load is jammed or the system stops for some reason the pump will not continue to pressurize the outlet line to the point where it could burst.
It has further been suggested to control such a pump by means of a volume sensor which detects the rate at which fluid flows through the outlet line and which depressurizes the pilot port of the pump when this rate exceeds a predetermined level. Such an arrangement is useful in that if the system springs a leak excessive fluid loss will be prevented.
The known systems have, however, the disadvantage that they are not able to respond fully to the various contingencies that can arise in a hydraulic system wherein the pump is connected to a load. Furthermore, they are frequently very complicated to adjust and are constituted by extremely complex mechanism which is highly failure-prone. What is more the known systems often are continuously adjusting or hunting so that the pump output is constantly varying, creating a strain on the entire system which can lead to its premature failure.